feat(i18n): translate remaining components (HU/EN/DE)

Feed, VideoCard, Sidebar, PlayerModal, Channels, Stats, SettingsPanel,
OnboardingWizard, NotificationCenter, Toaster, ErrorBoundary and the relativeTime
helper are now fully translated in Hungarian, English and German, each with its own
locale area file (auto-loaded). Key parity verified across all three languages.

frontend/src/i18n/locales/en/sidebar.json

@@ -0,0 +1,65 @@
+{
+  "filters": "Filters",
+  "activeCount": "{{count}} active",
+  "clearAll": "Clear all",
+  "resetDefaults": "Reset to defaults",
+  "customize": "Customize sidebar",
+  "done": "Done",
+  "editHint": "Drag to reorder · eye to show/hide",
+  "channel": "Channel",
+  "loading": "Loading…",
+  "thisChannel": "This channel",
+  "channelCount": "{{count}} channel",
+  "channelCount_other": "{{count}} channels",
+  "dragToReorder": "Drag to reorder",
+  "showWidget": "Show widget",
+  "hideWidget": "Hide widget",
+  "expand": "Expand",
+  "collapse": "Collapse",
+  "any": "Any",
+  "all": "All",
+  "tagModeTooltip": "Match any vs all selected tags",
+  "custom": "Custom",
+  "from": "From",
+  "to": "To",
+  "clearDates": "clear dates",
+  "widget": {
+    "show": "Show",
+    "sort": "Sort",
+    "date": "Upload date",
+    "content": "Content type",
+    "language": "Language",
+    "topic": "Topic"
+  },
+  "show": {
+    "unwatched": "Unwatched",
+    "in_progress": "In progress",
+    "all": "All",
+    "watched": "Watched",
+    "saved": "Saved",
+    "hidden": "Hidden"
+  },
+  "sort": {
+    "newest": "Newest",
+    "oldest": "Oldest",
+    "views": "Most viewed",
+    "duration_desc": "Longest",
+    "duration_asc": "Shortest",
+    "title": "Name (A–Z)",
+    "subscribers": "Channel subscribers",
+    "priority": "Channel priority",
+    "shuffle": "Surprise me"
+  },
+  "content": {
+    "normal": "Normal",
+    "shorts": "Shorts",
+    "live": "Live / Upcoming"
+  },
+  "datePreset": {
+    "24h": "24h",
+    "1week": "1 week",
+    "1month": "1 month",
+    "6months": "6 months",
+    "1year": "1 year"
+  }
+}